Laser-equipped machine tools are often used to cut parts from sheet metal and relatively thin plate. In such machine tools a laser beam concentrated by a focusing lens or mirror to a small diameter spot, is directed to position the focal point above, on or below the surface of the material to be cut. The laser beam is directed from the focusing optic through a nozzle disposed immediately above the material workpiece, with a pressurized gas being directed through the nozzle, typically coaxially with the laser beam, to assist making the cut. The pressurized gas interacts with the laser beam and material, facilitating the cutting process, and creates a gas stream which carries the removed material away from the cut.
Laser-equipped machine tools are Computer Numerically Controlled and are manufactured in many configurations and sizes and with lasers of various types and power. In one configuration, "flying optics", the cutting head is adapted for movement along one axis, such as the Y-axis which is mounted on a bridge, and the bridge is adapted for movement in an orthogonal, X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce and cut the workpiece to form holes and shapes in the material, then to cut the part from the material. In this configuration the laser is mounted on the stationary machine base or on a separate floor mounted stand.
When using laser-equipped cutting machine tools it is advantageous to utilize optics with different focal lengths to cut various thicknesses of material. The focal length of the optic contributes to the diameter of the focal spot and thus the energy density, Watts per unit area, at the focal spot. Shorter focal length optics create smaller focal spots having higher energy densities. The focal length of the optic also contributes to depth of focus of the focal spot with longer focal lengths having greater depth of focus. Shorter focal length optics are advantageous for cutting thinner materials while longer focal length optics are advantageous for cutting thicker material. Primarily the focal length of the optics and the power level of the laser contribute to the energy density remaining in the laser beam at distances beyond the workpiece.
Many same or different parts of common thickness and material type may be cut from a sheet or plate. Such groups of parts are commonly referred to as a nest. Left over material, after the parts have been removed, is referred to as a remnant or a skeleton. A small remnant which falls from a hole cut in a part is called a slug. Remains of material from the cut is called slag. Resolidified material clinging to the part is called dross. The mixture of slugs and slag residue from cutting sheet material is generally called scrap.
Various means for collecting and removing scrap from laser cutting machines have been utilized. One version is to allow the scrap to accumulate on the floor or on a platform or bed disposed below the cutting area. When the accumulation is excessive it is shoveled out. This method is advantageously low cost. It also has disadvantages. The machine must be shut down while the scrap is removed, reducing productivity. Debris falling from the shovel, can land on way covers or machine parts, where not wanted, leading to premature failures.
Another version is to provide one or more scrap collecting pans under the cutting area to collect the scrap. This solution is also advantageously low cost. It also has disadvantages. The machine is normally shut down while the scrap is removed, reducing productivity. If an excessive amount of scrap is allowed to accumulate, the pans are very difficult to remove. The scrap pans may be large and hard to handle.
Another version is to provide a conveyor disposed below the cutting area to carry or drag the scrap from the machine. While this arrangement costs more, it also improves machine productivity by eliminating machine shut down for removal of scrap. Conveyor systems, however, can increase the height of the bed by an unacceptable amount, and are also prone to damage.
As lasers, with beam qualities suitable for cutting, are developed and become available in higher powered versions, machines are developed having ability to cut greater thicknesses of material. Adapting high power lasers to cut thicker materials leads to using focusing lenses with longer focal lengths. Below the focal point, a laser beam expands at approximately the same rate that it was focused. For example, if a 35 mm diameter laser beam is focused by a lens with a 10" focal length, then, 10" below the focal point, unless absorbed by the material cut, the beam would be about 35 mm in diameter again. Twenty inches below the focal point the beam would be about 70 mm in diameter. This remnant diverging beam from high power lasers have considerable capability to cause damage. For example in testing leading to the present invention, a 0.125" thick aluminum plate was scuffed with steel slag, then a 38 mm diameter 5500 Watt beam was directed at the surface. The aluminum was cut through in 40 seconds. Similar tests were made with 0.25" inch thick stainless steel and carbon steel. Both were cut through in well under a minute. These tests indicated that a scrap collection bed underlying the cutting area of a high power laser system, with long focal length optics in use, would be at considerable risk of being damaged by the remnant laser beam.